The OASIS Open Document Format for Office Applications (OpenDocument) is a standardized XML-based file format specification suitable for office applications. It covers the features required by text, spreadsheets, charts, and graphical documents.
It is maintained by a vendor neutral organisation and has no licence restrictions for implementation in any kind of product. It is an OpenStandard, in that it is believed to be possible to create implementations of the standard which are free from patent, trademark and copyright encumberances. Some are predicting it may form the basis of the next generation of the web page format, although this seems unlikely, because it has no explicit security model.
For more detailed information, see the the OASIS pages: http://www.oasis-open.org/committees/tc_home.php?wg_abbrev=office
Those interested in working with OpenDocument XML may wish to read, OASIS OpenDocument Essentials: Using OASIS OpenDocument XML, a work in progress by J. David Eisenberg for O’Reilly & Associates (currently available under a Creative Commons License).
There is also a group who are independently promoting this format, known as the OpenDocumentFellowship.